Amorphous silica filler

ABSTRACT

The amorphous silica has a large apparent specific gravity despite of its BET specific surface area of 200 to 500 m 2  /g, has an oil absorption amount of as small as 50 to 120 ml/100 g, has an agglomeration degree (D1/D0=DA) defined by the ratio of silica elementary particle diameter D0 to primary particle diameter D1 of from 10 to 50, and has nearly a spherical particulate structure which is quite different from that of the conventional gelation method silica (DA=OO). The amorphous silica-type filler exhibits excellent properties in regard to handling, processability, dispersion property and abrasion-resistant property and, when added as, for example, an anti-blocking agent, makes it possible to obtain resin films having transparency and scratch-resistant property without fisheyes or voids. Owing to the above properties, the amorphous silica that is used as a filler for the heat-sensitive recording papers suppresses ground fogging, improves the image concentration, makes it possible to apply a highly viscous coating solution since the filler dispersed in an aqueous solution creates very small viscosity, and imparts excellent abrasion resistance to the apparatus during the step of coating.

This is a division of application Ser. No. 08/095,245, now U.S. Pat. No.5,384,191, filed Jul. 22, 1993 which is a division of Ser. No.07/851,910 filed Mar. 16. 1992, now abandoned U.S. Pat. No. 5,266,397.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an amorphous silica filler, and morespecifically to an amorphous silica filler which exhibits excellenthandling property and processability and which, when added to a resinfilm, exhibits excellent dispersing property, transparency andanti-blocking property, and which makes it possible to obtain a resinfilm having excellent scratch-resistant property that does not getscratched on the surfaces even when the films are rubbed by each other.

The present invention further relates to a filler for heat-sensitiverecording papers, and more specifically to an amorphous silica fillerfor heat-sensitive recording papers that can be excellently applied ontothe heat-sensitive recording papers, that makes it possible to form avivid heat-sensitive image without fogging on the ground, and that doesnot permit scum to adhere on the thermal head.

2. Description of the Prior Art

The amorphous silica fillers include the so-called dry-method silica andthe wet-method silica, and are used for paints, information-recordingpapers, rubbers, resin molded products and like applications dependingupon their properties.

The amorphous silica exhibits properties that vary greatly dependingupon the manufacturing method. In particular, the latter wet-methodamorphous silica exhibits widely varying properties if conditions suchas concentration, temperature, pressure, time and reaction method arechanged in neutralizing an alkali silicate with an acid.

For instance, there have been known a silica hydrogel (Japanese PatentPublication No. 13834/1973, Japanese Patent Laid-Open No. 16049/1988)obtained by gelatinizing (spraying into a gaseous medium) within a shortperiod of time a silica sol formed by neutralizing an alkali silicatewith an acid, and a hydrated silica gel (Japanese Patent Publication No.1764/1990) having a water content of 20 to 50% by weight obtained bywashing with an acid a silica hydrogel hardened with an acid to obtain asilica hydrogel having pH of 2.5 to 5, followed by drying andpulverizing.

In addition to the above amorphous silica, furthermore, there haveheretofore been widely used inorganic fine particles as an anti-blockingagent for resin films. For instance, there have been proposed a method(Japanese Patent Publication No. 41099/1974) of improving transparencyand anti-blocking property by adding fine powdery silica whose silanolgroups on the surfaces thereof have been substituted by oleophilicgroups to a resin, a method (Japanese Patent Publication No. 16134/1977)of improving anti-blocking property by adding zeolite particles tobiaxially undrawn polypropylene film, and a drawn polypropylene film(Japanese Patent Publication No. 58170/1988) having improvedtransparency, slipping property and anti-blocking property obtained byadding fine powdery silica having an apparent specific gravity of 0.1 to0.2 g/cm³ and a specific surface area of smaller than 150 m/g.

Furthermore, a heat-sensitive recording paper has heretofore been usedin facsimiles, printers, data communications, computer terminals,measuring instruments and copying machines which use a thermal head, aheat pen, an infrared-ray lamp or a laser as a source of heat, theheat-sensitive recording paper being provided with a recording layerthat comprises a color agent such as a leuco-pigment and a color couplersuch as phenols that develop a color upon hot-contact with the coloragent, which are contained in a binder.

In making a record on the heat-sensitive recording paper by the heat bybringing a recording head into contact with the recording layer, thecomponents in the recording layer melt and stick resulting in theadhesion of scum onto the recording head and the development ofsticking. In order to prevent these problems, various fillers arecontained in the recording layer. When the amorphous silica is containedin the recording layer as a filler for the heat-sensitive recordingpaper, however, the reaction is promoted between the leuco-pigment andthe phenols due to the surface activity of silica giving rise to thedevelopment of ground color (ground fogging).

In order to prevent the above problem, the present inventors haveproposed in Japanese Patent Publication No. 1030/1990 a filler forheat-sensitive recording papers comprising fine particulate amorphoussilica having a particle size distribution of secondary particle sizesin which particles of a size of smaller than 4 μm as measured by thecentrifugal sedimentation method are contained in amounts of more than90% by weight of the total weight, having a BET specific surface area of10 to 100 m² /g, and having a bulk density of 0.14 to 0.30 g/ml.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an amorphous silicafiller which exhibits excellent handling property, processability, anddispersion property, and which causes less damage to the apparatus thatresults from abrasion. The invention further provides an amorphousanti-blocking agent for resin films that imparts excellent anti-blockingproperty (AB property) to the resin film, that exhibits excellentdispersion property and transparency, and that imparts excellentscratch-resistant property to the film surfaces so that they will notget scratched when the films are rubbed by each other, and provides athermoplastic resin film using the above anti-blocking agent.

Amorphous silica has heretofore been widely used as an anti-blockingagent for resin films because of the reason that the resin films blendedwith amorphous silica exhibits excellent anti-blocking property andtransparency leaving, however, room for improvement such as insufficientdispersion property and film defects such as fisheyes and voids. It hastherefore been desired to remove these defects and to further improveanti-blocking property and transparency.

The reason why the film blended with amorphous silica exhibits improvedanti-blocking property is attributed to that the amorphous silicaparticles distributed on or near the surface of the film form a gapbetween the film surfaces. When the films are rubbed by each other,however, there arises a big problem in that the film surfaces getscratched by the grinding action of amorphous silica. In particular, thehigher the anti-blocking property of the amorphous silica, the more thesurface of the film tends to be get scratched by abrasion.

Moreover, the conventional amorphous silica used for the above-mentionedapplications is in a fine powdery form having a small apparent specificgravity and large bulkiness, and lacks handling property because it isso powdery. Besides, the amorphous silica is so bulky that it lacksprocessability and dispersion property when it is blended together withthe resin.

The present inventors have found through the study such experimentalrules between the specific surface area of amorphous silica and thesurface hardness that Mobs' hardness decreases with a decrease in thespecific surface area and that Mobs' hardness increase with an increasein the specific surface area. Moreover, the amorphous silica having alarge specific area includes particles having appearance of the shape ofbroken glass pieces and sharp edges. This is attributed to that in theamorphous silica having a large specific surface area, the particle size(which in this specification is referred to as elementary particle size)becomes very fine when the soluble silicic acid component is liberatedin the form of silicic acid, and the elementary particles firmlycoagulate together to form gel-like particles.

Depending upon the AB property and the transparency of the film,however, it often becomes necessary to use amorphous silica having arelatively large specific surface area for the resin film, resulting inthe development of troubles such as damage to the apparatus by abrasionduring the production of resin films, and development of scratches onthe film surfaces as the product films are rubbed by each other,deteriorating the commercial value of the products. Therefore, nosatisfactory amorphous silica has yet been obtained that can be used asan anti-blocking agent for the resin films.

Another object of the present invention is to provide a filler forheat-sensitive recording papers that does not develop ground fogging,that exhibits excellent scum-adhesion preventing property, and that iscapable of forming image of a high concentration.

A further object of the present invention is to provide a filler forheat-sensitive recording papers that is capable of forming a dispersionof a high concentration at the time of coating operation, and that helpsfacilitate the coating operation and decrease the manufacturing cost.

That is, the fine particulate amorphous silica proposed in JapanesePatent Publication No. 1030/1990 is a very excellent one for solving theproblems of the heat-sensitive recording papers, but has a fine particlesize and causes the viscosity of the coating solution thereof to becomevery great. When the amorphous silica is used as a coating filler,therefore, the coating operation must be carried out by selecting thefiller concentration of the coating solution to be lower than that ofwhen calcium carbonate or baked kaolin is used. Moreover, an extendedperiod of time is required for drying giving problems from the viewpointof coating operation and cost of producing recording papers.

According to the present invention, there are provided an amorphoussilica filler having an average primary particle diameter (D1) of 100 to270 nm as observed by a scanning-type electron microscope, an apparentspecific gravity (JIS K 6220.6.8 method) of 0.24 to 0.55 g/cm³, a BETspecific surface area of 200 to 500 m² /g, a silica elementary particlediameter (DO) of 5 to 15 nm as calculated from the BET specific surfacearea, and an agglomeration degree (DA) defined by the ratio (averageprimary particle diameter (D1))÷(silica elementary particle diameter(DO) of from 10 to 50, as well as a filler for heat-sensitive recordingpapers.

According to the present invention, furthermore, there is provided athermoplastic resin film obtained by blending the amorphous silica as ananti-blocking agent in an amount of 0.01 to 5 parts by weight per 100parts by weight of the thermoplastic resin.

According to the present invention, there is further provided aheat-sensitive recording paper obtained by adding the amorphous silicaas a filler in an amount reckoned as a solid component of 5 to 60% byweight into a heat-sensitive recording layer composition of aheat-sensitive recording paper that is known per se.

Various particle sizes of the amorphous silica will be referred to inthis specification, and the method of measurement and meanings are asdescribed below.

If the BET specific surface area (m² /g) is denoted by SA and theelementary particle size (nm) by DO, then the silica elementary particlediameter (DO) is calculated from a relation SA=2727÷DO. The specificsurface area of the amorphous silica particles varies depending upon aminimum basic particle diameter of when the amorphous silica particlesare precipitated as free silicic acid, i.e., varies depending upon theelementary particle diameter. The amorphous silica elementary particlesare essentially of a spherical shape, and do not exist alone, and theirpresence cannot be confirmed by an ordinary electron microscope.

Average primary particle diameter (D1): The individual particlediameters are measured from a scanning-type electron microphotography ofamorphous silica, and a number average value thereof is found. A minimumparticle diameter of amorphous silica that consists of agglomerated orcoagulated elementary particles and that can be detected through anelectron microscope.

Secondary particle diameter (D2): A median diameter based on the volumeby measuring amorphous silica by the Couter Counter method. A particlediameter with which the amorphous silica consisting of the coagulationof the primary particles is allowed to behave as powdery particles.

According to the present invention, the amorphous silica has an averageprimary particle diameter (D1) of from 100 to 270 nm yet has a BETspecific surface area of 200 to 500 m² /g, and further has anagglomeration degree (DA) defined by D1/DO of from 10 to 50, making adistinguished feature.

FIG. 1 is a scanning-type electron microphotography showing the particlestructure of amorphous silica used in the present invention, from whichit will be understood that the amorphous silica used in the presentinvention has an average primary particle diameter of 100 to 270 nm, aclear particle shape (appearance), and a uniform particle structure. Itwill further be recognized that the primary particles are distributedsymmetrically.

The amorphous silica of the present invention has a particle structurewhich is radically different from the conventional amorphous silica thathas silica elementary particles finer than those of the presentinvention, that is not observed as so-called monotonous particles by anordinary electron microscope but is observed as discrete particleshaving ambiguous agglomeration degree (see FIG. 2), or that is observedin the form of a continuous and homogeneous gel (see FIG. 3).

According to the present invention, the BET specific surface area isspecific to range from 200 to 500 m² /g because when the specificsurface area becomes smaller than 200 m² /g, the anti-blocking propertyis lost and the film defects develop such as fisheyes and voids and whenthe specific surface area becomes greater than 500 m² /g, the haze ofthe film increases, the gloss decreases, transparency is lost, the filmgets scratched by abrasion in increased amounts, and it becomesdifficult to set the average primary particle diameter to lie within arange of the present invention no matter how the powder isafter-treated.

Amorphous silica having the BET specific surface area of from 200 to 500m² /g has been widely known. According to the present invention,however, particularly important feature resides in that theagglomeration degree (DA) of silica elementary particles lies from 10 to50, and the average primary particle diameter (D1) lies from 100 to 270nm.

According to the present invention in which the agglomeration degree(DA) and the average primary particle diameter (D1) are maintained tolie over the above-mentioned ranges, the film exhibits excellentanti-blocking property as well as excellent resistance against scratchesby abrasion when the films are rubbed by each other. These facts willbecome readily obvious from the examples and comparative examplesdescribed later.

By setting the agglomeration degree (DA) and the average primaryparticle diameter (D1) to lie within the above-mentioned ranges,furthermore, it is allowed to form a vivid heat-sensitive imagemaintaining a high level without ground fogging on the heat-sensitiverecording paper, and to greatly improve resistance against abrasion ofthe apparatus during the step of coating. This fact can be readilyunderstood from FIG. 1 which shows the particle structure of amorphoussilica of the present invention and from FIG. 3 which shows the particlestructure of conventional amorphous silica in which the particles havevitreous appearance.

Because of such a novel particle structure, the amorphous silica used inthe present invention does not form vitreous gel: large silica particlesof a primary particle diameter that serve as nuclei are agglomerated byvery fine silica particles that serve as a binder, and wherebyrelatively large apparent specific gravity and BET method specificsurface area are exhibited, and the oil absorption amount is as small as50 to 120 ml/100 g.

Therefore, the amorphous silica of the present invention has a particlestructure consisting of large primary particles despite of its large BETspecific surface area. When dispersed in, for example, water, therefore,the dispersion exhibits a very small viscosity.

Referring to FIG. 4 showing a relationship between the dispersionconcentration of filler in water and the viscosity, the conventionalamorphous silica filler (specific surface area 60 m² /g, apparentspecific gravity 0.18 g/cm³) exhibits viscosity that abruptly increasenear the dispersion concentration in water of about 30% by weight, i.e.,exhibits very increased viscosity, whereas the filler of the presentinvention exhibits viscosity that rises very mildly with respect to thedispersion concentration in water. Therefore, the filler of the presentinvention can be provided in the form of a dispersion solution of a lowviscosity yet having a high concentration, and contributes to enhancingthe coating operation efficiency and decreasing the energy cost forevaporating water.

The amorphous silica of the present invention has the particle structureas described above in which the secondary particle diameter is as smallas 1 to 5 μm and, preferably, 1 to 2.5 μm, and is very dense andexhibits very excellent dispersing property. Therefore, the amorphoussilica of the present invention can be used for the undercoating layeror the color recording layer of the heat-sensitive paper to obtain acoated surface having excellent smoothness.

Moreover, the primary particles are coagulated little, the secondaryparticle diameter is from 1 to 5 3/4m as measured by the Coulter Countermethod, and the amorphous silica disperses excellently into the resin.Moreover, the film that is formed develops no fisheyes or voids andexhibits excellent appearance.

When used for the heat-sensitive recording paper, furthermore, theamorphous silica of the present invention exhibits satisfactory effectsfor preventing adhesion of scum and for preventing sticking though theoil absorption amount is as small as 50 to 120 ml/100 g, and maintainsfavorable matching property with respect to the thermal head This is dueto the fact that the amorphous silica so far had to be highly oilabsorption (100 ml/100 g or greater) but the heat-sensitive recordingpapers nowadays have a three-layer structure that includes theundercoated layer and need not be so highly oil absorption. That is,when a pigment having oil absorption property which is as great as 200ml/100 g is used for the undercoating layer, the binder that isnecessary for the color developing layer is absorbed too at the time ofapplying the heat-sensitive coating solution. The amorphous silica ofthe present invention which is modestly oil absorption does not absorbthe binder excessively and helps increase the strength of the surface ofthe heat-sensitive recording papers. Moreover, when used for theundercoated layer or the color developing layer of not only theheat-sensitive paper of the two-layer structure but also of theheat-sensitive paper of the three-layer structure, the amorphous silicaof the present invention exhibits the effect for preventing the adhesionof scum or for preventing sticking.

Moreover, the amorphous silica of the present invention has a very smallporous diameter despite of its specific surface area which is as largeas 200 to 500 m² /g as measured by the BET, and makes it possible toobtain a heat-sensitive recording paper without ground fogging. Forexample, the white carbon or the gel-type silica having a large specificsurface area develops ground fogging when the heat-sensitive coatingsolution is being prepared or when the heat-sensitive recording paper isprepared or after it is prepared because of the reason that theleuco-dye molecules enter into relatively large pores that work as asolid acid-type developer. The amorphous silica of the presentinvention, on the other hand, has a particular particle structure and asmall oil absorption amount despite of its relatively large BET specificsurface area. It is therefore considered that the amorphous silica ofthe present invention has a very fine porous diameter and does notabsorb leuco-dye molecules.

It has further been empirically known that there exists such arelationship between the specific surface area and the surface hardnessof the amorphous silica that the amorphous silica having a largespecific surface area usually has Mohr's hardness of 2 or greater.According to this relationship, it is expected that the amorphous silicaof the present invention has a large specific surface area, is dense,and has a large hardness, and causes the thermal head to be worn. As isobvious from FIG. 1, however, the amorphous silica used in the presentinvention is quite round in appearance and exists as groups of edgelesscoagulations and does not cause the thermal head to be worn out.Moreover, the amorphous silica of the present invention has a highpurity and does not cause the thermal head to be corroded.

In relation to the primary particle structure mentioned above,furthermore, the amorphous silica used in the present invention has anadditional feature in that it has an oil absorption amount which is assmall as 50 to 120 ml/100 g yet having a relatively large BET specificsurface area. In blending the filler or the pigment into the resin, ingeneral, the surface are wet well as the specific surface area becomessmall, and the volume being charged becomes small and is easily blendedas the apparent specific gravity becomes great. The amorphous silica ofthe present invention has a large apparent specific gravity and a smalloil absorption amount, and can be highly densely applied as aheatsensitive recording layer composition, make it possible to prepare acoating solution that can be favorably applied even with a small amountof binder, and can further be blended into. the resin and uniformlydispersed into the resin.

The important feature of the amorphous silica used in the presentinvention is that the apparent specific gravity (JIS K 6220.6.8 method)lies within a range of from 0.24 to 0.55 g/cm³. The amorphous silicahaving a large apparent specific gravity has a coarse primary particlediameter (amorphous silica of the sedimentation method). On the otherhand, the amorphous silica having fine primary particle diameter has anapparent specific gravity which is smaller than 0.2 g/cm³. However, theamorphous silica of the present invention is more dense than theconventional amorphous silica of the sedimentation method, and has anapparent specific gravity which is nearly comparable to that of theamorphous silica of the gel method. The amorphous silica used in thepresent invention is dense and is prevented from becoming powdery,exhibits excellent handling property, can be easily blended and kneadedwith the resin, and can be processed well. Moreover, the powderyparticles are dense and does not easily fly into the air, and does notdeteriorate the working environment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning-type electron microphotography (magnification of20,000 times) showing the particle structure of an amorphous silicafiller of the present invention:

FIG. 2 is a scanning-type electron microphotography (magnification of20,000 times) showing the particle structure of a conventional amorphoussilica (Comparative Example 2);

FIG. 3 is a scanning-type electron microphotography (magnification of20,000 times) showing the particle structure of a conventional amorphoussilica (Comparative Example 3); and

FIG. 4 is a diagram illustrating a relationship between the dispersionconcentration of a filler in water and the viscosity wherein a curve Ais that of when the amorphous silica (Example 1) of the presentinvention is used, a curve S is that of when a baked kaolin is used, acurve C is that of when the conventional amorphous silica (ComparativeExample 2) is used,and a curve D is that of when a light calciumcarbonate is used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Amorphous Silica)

According to the present invention, the amorphous silica has an averageprimary particle diameter (measured by using a scanning-type electronmicroscope) of 100 to 270 nm, has an average secondary particle diameteras measured by the Coulter Counter method of from 1 to 5 μm, preferablyfrom 1.5 to 4.5 μm, and particularly preferably from 1.8 to 3 μm, has anapparent specific gravity (JIS K 6220.6.8 method) of from 0.24 to 0.55g/cm³, preferably from 0.27 to 0.5 g/cm³, and particularly preferablyfrom 0.3 to 0.45 g/cm³ has an oil absorption amount (JIS K 5105 method)of from 50 to 120 ml/100 g, preferably smaller than 100 ml/100 g andparticularly preferably smaller than 80 ml/100 g, has a BET specificsurface area (SA) of from 200 to 500 m² /g, and an agglomeration degree(DA) defined by the ratio of (average primary particle diameter(D1))÷(silica elementary particle diameter (D0)) of from 10 to 50,preferably from 15 to 45, and particularly preferably from 20 to 40.

(Preparation Method)

Though not limited to the below-mentioned method, the amorphous silicaused in the present invention is prepared by a method which can belocated midway between the conventional gelation method and thesedimentation method and, more concretely, by reacting an acid sol inthe presence of sodium silicate and an aqueous solution of salts underparticular conditions. In preparing the amorphous silica of the presentinvention, first, it is desired to prepare an acid silica sol in whichthe equivalent amount of silica component put to the reaction has a pHof 0.2 to 2.5 and the concentration of silica reckoned as SiO₂ is from 3to 20% by weight.

The acid silica sol is obtained by adding an aqueous solution of sodiumsilicate of an amount of 20 to 70% of the total weight of sodiumsilicate to an aqueous solution of hydrochloric acid or sulfuric acid of10 to 60% by weight with stirring, such that the pH value after thereaction lies within the below mentioned range. Then, the acid silicasol is added to the remaining aqueous solution of sodium silicate inwhich table salt is dissolved so that the amount of silica reckoned asSiO₂ is 10 to 100% by weight with stirring at 5° to 90° C., in order toobtain a silica hydrogel of the present invention.

Here, it is important that the solution is sufficiently stirred so thatthe pH value will be from 5 to 8, and preferably from 6 to 7 after thereaction.

Furthermore, the reaction method for preparing the silica hydrogel ofthe present invention is in no way limited to the above-mentioned methodonly, but may be the one in which the acid silica sol is prepared, tablesalt is made present therein in an amount of 10 to 300% by weight of theamount of SiO₂, and the remaining aqueous solution of sodium silicate isadded thereto, so that the pH value lies between 5 to 8 after thereaction.

Then, the silica hydrogel is filtered and washed with water by anordinary method, dried at a temperature of 120° to 400° C., and is, asrequired, classified to obtain a product of the invention.

The amorphous silica obtained according to the present invention has anSiO₂ content of 90 to 96% by weight, and preferably 91 to 95% by weightunder the condition of being dried at 150° C.

The amorphous silica filler used in the present invention is prepared bythe method described above. If the amorphous silica obtained byneutralizing or salting-out the acid silica sol is referred to as thesedimentation-method silica and if the amorphous silica obtained byneutralizing the sodium silicate is referred to as the gel-methodsilica, then the amorphous silica of the present invention can be saidto be comprised of both the sedimentation-method silica and thegel-method silica. Usually, the sedimentation-method silica and thegel-method silica are mixed together in the form of elementaryparticles. It will, however, be obvious for people skilled in the artthat they may have a composite particulate structure in which either oneor both of them have the form of nuclear particles, and the surfaces ofthe nuclear particles are covered with shells (coatings) of thesedimentation-method silica or the gel-method silica.

When the amorphous silica of the present invention is used as ananti-blocking agent for resins, other organic components may be carriedor blended at the time of molding the resin, such as lubricating agent,antistatic agent, plasticizer, nucleating agent, de-fogging agent,ultraviolet-ray absorbing agent, anti-oxidizing agent, insecticidalagent, fungicidal agent, perfumes, coloring agent medicinal agent, andlike components. Moreover, the amorphous silica can be used beingblended with other inorganic fillers. Furthermore, the filler as well asthe above organic components may have been surface-treated in advancewith various metal soaps, waxes, resin, surfactant, lubricating agent,various coupling agents, or inorganic oxides.

The organic components and inorganic components that are carried orblended or are used for the surface-treatment, should be in amounts of0.1 to 30% by weight and, particularly in amounts of 1 to 10% by weightwith respect to the amorphous silica.

(Applications)

The order to impart transparency, slipping property and anti-blockingproperty, the amorphous silica of the present invention can be used forthe resin molded products such as a variety of drawn films, e.g., usedbeing blended in a variety of thermoplastic resins like a homopolymer ofpropylene which is a crystalline propylene copolymer or anethylene-propylene copolymer; low-, medium- an high-density or linearlow-density polyethylene; olefin-type resins such as ionicallycross-linked olefin copolymer, ethylene-vinyl acetate copolymer,ethylene-acrylic acid ester copolymer, or the like; thermoplasticpolyesters such as polyethylene terephthalate, polybutyleneterephthalate, and the like; polyamide resins such as 6-nylon,6,6-nylon, and 6,8-nylon; chlorine-containing resins such as vinylchloride, vinylidene chloride, and the like; polycarbonates; andsulfones.

To accomplish the above object, the amorphous silica of the presentinvention can be blended in an amount of 0.01 to 10 parts by weight, andparticularly in an amount of 0.1 to 5 parts by weight per 100 parts byweight of the thermoplastic resin.

By utilizing the above-mentioned properties, furthermore, the amorphoussilica of the present invention can be contained as a filler forheat-sensitive recording paper in a heat-sensitive recording layercomposition which is known per se in an amount of 5 to 60% by weight,and particularly in an amount of 20 to 40% by weight on the basis ofsolid component.

In this composition, examples of the leuco pigment which is a coloragent includes triphenylmethane-type leuco pigment, fluoran-type leucopigment, spiroran-type leuco pigment, fluoran-type leuco pigment,spiroran-type leuco pigment, Rhodamine lactam-type leuco pigment,Auramine-type leuco pigment, and phenoliazine-type leuco pigment, whichcan be used alone or in a combination of two or more for theheat-sensitive recording papers of this kind.

Any phenol can be used as a color coupler, such as bisphenol A,hisphenol F, or 2,6-dioxybenzoic acid, which is in a solid form atnormal temperature and melts upon heating.

It is further allowable to use, as auxiliary components, calciumcarbonate baked kaolin, aluminum hydroxide, and silicates such asaluminum silicate, calcium silicate and magnesium silicate which arewidely known fillers, being mixed in the silica of the presentinvention.

As a binder, furthermore, there may be used any water-soluble resin orwater-dispersing resin, such as starch, cyanomethyl starch, carboxylstarch, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinylalcohol, water-soluble acrylic resin, vinyl methyl-ether copolymer,sodium alginate, SBR latex, NBR latex, or ethylene-vinyl acetatecopolymer.

As sensitizing agent, furthermore, there can be blended various waxes,such as fatty acid, fatty acid amide, carnauba wax, polyethylene wax, ororganic bases such as alkanolamine for preventing the development ofground color.

To form the heat-sensitive recording layer, a dispersion solution ofleuco pigment dispersed in a binder solution and a dispersion solutionof phenols dispersed in a binder solution are prepared, and are appliedonto a paper or a synthetic paper. The amorphous silica filler of thepresent invention can be mixed in advance in a dispersion solution ofphenol and there further may be separately prepared a dispersionsolution of the amorphous silica filler dispersed in a binder solution,which are then mixed into the above two dispersion solutions to form theheat-sensitive recording layer.

Moreover, the amorphous silica filler of the present invention can beblended in the heat-sensitive layer or can be used as the undercoatedlayer being applied onto the heat-sensitive layer. As will be obviousfrom the scanning-type electron microphotography of FIG. 1, the fillerfor heat-sensitive recording paper of the present invention has nearlyspherical appearance with uniform particle diameters. When used as theundercoated layer, therefore, the ground is uniformalized contributingto greatly improving the smoothness of the recording layer that is a toplayer.

Moreover, the amorphous silica of the present invention can be used fora variety of applications being blended in various paints, adhesives,and coating resin compositions, and can be blended as a filler inmedicines, foods, agricultural chemicals and insecticides.

Being used in combination with calcium carbide, furthermore, theamorphous silica filler of the present invention can be applied as a topcoated layer to serve as a delustering agent for tracing papers, awritability improving agent, a writability improving agent for syntheticpapers, color improving agent for diazo photosensitive papers, and asticking agent for inks.

EXAMPLES

The invention will now be concretely described by way of the followingexamples. Here, in the present invention, the amorphous silica wastested for its properties, and the anti-blocking agent and the fillerfor heat-sensitive paper were tested and evaluated in a manner asdescribed below.

(1) BET specific surface area.

The nitrogen gas-adsorption amount (cc/g) adsorbed by the surface of asample as a single molecular layer if found by using an automatic BETspecific surface area measuring instrument (manufactured by CARLO-EBRACo., Sorptomatic Series 1800), and the BET specific surface area iscalculated from a specific surface area (SA)=4.35×Vm (m^(2/) g).

(2) Average secondary particle diameter (median diameter).

A particle diameter found from a 50% point of volume distribution on acumulative particle size curve obtained by the Coulter Coulter method(manufactured by Coulter Electronics Co. of U.S.A., Model TA-II).

(3) Primary particle diameter.

Particle diameters (nm) in a limited visual field image arearithmetically averaged to find an average primary particle diameter byusing a scanning-type electron microscope WET-SEM (WS-250) manufacturedby Akashi Beam Technology Co. of Japan. (4) Elementary particlediameter.

It has been clarified by R.K. Iler^(*) that there exists the followingrelationship between the BET specific area SA (m² /g) and the elementaryparticle diameter DO (nm), and DO (nm) is calculated from therelationship SA=2727/DO.

(5) Agglomeration degree (DA).

The agglomeration degree DA of silica elementary particles constitutingprimary particles is defined by the formula,

DA=(average primary particle diameter (D1))÷(silica elementary particlediameter (DO))

(6) Oil absorption

The oil absorption amount is found on compliance with JIS K 5101-19 thatspecifies a pigment testing method.

(7) pH Value.

The pH value is found in compliance with JIS K 5101-24A that specifiespigment testing method.

(8) Apparent specific gravity.

The apparent specific gravity (g/cm³)is found in compliance with JIS K6220.6.8 that specifies method of testing blending agents for rubbers.

(9) Preparation of films.

Undrawn polypropylene film (C-PP film)

C-PP films are obtained under the following conditions by using twokinds of polymers, i.e., a homopolymer of a propylene (melt flow rateMFR=1.8 g/10 min., isotactic index I.I.=96.0) as a crystallinepropylene-type polymer and an ethylene-propylene random copolymer (meltflow rate MFR=6.5 g/10 min isotactic index I.I.=97.0, ethylenecontent=4.0 mol %, DSC melting point=140° C. according to ASTM D-3417).

An amorphous silica powder is added as an anti-blocking agent(hereinafter referred to as AB agent) in an amount of 0.15 parts per 100parts of the propylene-type polymer. At the same time, 0.15 parts of a2,6-ditertiary butyl paracresol and 0.1 part of calcium stearate areadded as anti-oxidizing agents, and the mixture re mixed using theHenschel mixer at 1000 rpm for 3 minutes. The mixture is then melted andpelletized using a monoaxial extruder having a diameter of 65 mm. Thepellets are extruded by ab extruder of 35 mm in diameter having a T-diesat 230° C. to obtain an undrawn film (film sample F-1) having athickness of 25 μm.

Biaxially drawn polypropylene film (O-PP film)

An amorphous silica powder is added as an AB agent in an amount of 0.15parts per 100 parts of a homopolymer of a propylene (melt flow rateMFR=1.8 g/10 min., isotactic index I.I.=96.0) which is a crystallinepropylene-type polymer. At the same time, 0.15 parts of a 2,6-ditertiarybutyl paracresol and 0.1 part of calcium stearate are added asanti-oxidizing agents, and the mixture is mixed using the Henschel mixerat 1000 rpm for 3 minutes. The mixture is then melted and pelletizedusing the monoaxial extruder having a diameter of 65 mm. The pellets areextruded by an extruder of 35 mm in diameter having T-dies at 230° C. toobtain a sheet-like film 1250 μm in thickness. Using biaxially drawingmolding machine having a diameter of 115 mm, the film is drawn at 115°C. into five times in the vertical direction and is then drawn into tentimes in the lateral direction is a tenter oven at 170° C., in order toobtain a biaxially drawn film having a thickness of 25 μm (film sampleF-2).

Nylon film (O-NY film)

The amorphous silica powder is added as an AB agent in an amount of 5parts per 94 parts of a 6-nylon powder (granular diameter of 300 mesh orsmapper, specific gravity=1.14, melting point=220° C.). At the sametime, 1 part of ethylenebis-stearylamide is added as a dispersing agent,and the mixture is mixed by the Henschel mixer at 1000 rpm for 3minutes. The mixture is then melted at 260° C. by using a monoaxialextruder of a diameter of 65 mm to obtain master pellets containing 5parts of the AB agent. 2 Parts of the master pellets and 98 parts of the6-nylon pellets without containing AB agent are mixed together to obtainpellets containing 0.1 part of the AB agent. The mixture pellets arethen melt-extruded by an extruder of a diameter of 35 mm having aT-dies, and are solidified on cooling roll of 40° C. to obtain anundrawn film having a thickness of 140 μm. The film is biaxially drawnsimultaneously into three times in the vertical and lateral directionsat a drawing temperature of 120° C. by using a biaxially drawing molderhaving a diameter of 115 mm, and is then thermally set at 190° C. toobtain a biaxially drawn film having a thickness of 15 μm (film sampleF-3).

(10) Appearance of films.

Fisheyes or voids appearing on the surfaces of films of a size of 20cm×20 cm are observed by naked eyes, and are evaluated on the followingbasis.

⊚: No fisheye or void is observed.

◯: Fisheyes or voids are recognized on the areas less than about 10% ofthe whole film.

Δ: Fisheyes or voids are spreading on about 50% of the whole film.

X: There are fisheyes or voids in such large amounts as more than about50% of the whole film.

(11) Scratch resistance on the film surfaces.

Rolled films (200 m/roll) after the preparation are unwound three timesat a speed of about 20 m/min. and are evaluated on the following basis.

◯: The degree of scratch is very small or there is no scratch.

Δ: Scratch is apparently recognized but is not spreading on the wholesurface.

X: There are so many scratches that the film is not practically usable.

(12) Haze (%) of films.

The haze is measured in compliance with the method of ASTM-D 1003-61 byusing a directly read-type haze meter manufactured by Suga TestingMachine Co., Japan.

(13) Static coefficient of friction.

The static coefficient of friction (μs) and the dynamic coefficient offriction (μd) are measured in compliance with the method of ASTM-D 1894by using a slop tester manufactured by Nippon Rigaku Kogyo Co., Japanunder the following conditions.

Film sample F-1: 23° C., 40° C.×3 days

Film sample F-2: 40° C.×1 day

Film sample F-3: RH (relative humidity) 65%, RH 85%

(14) Anti-blocking property (AS property: g/cm)

Two pieces of films are superposed one upon the other so that thecontact area is 10 cm², sandwiched between two pieces of glass plates,and are left to stand at 40° C. for 24 hours under the application of aload of 50 g/cm². After left to stand, a maximum load with which theupper film and the lower film peel off from each other is measured usinga frictional coefficient measuring instrument manufactured by Toyo SeikiCo., Japan.

Film sample F-1: 50° C.×7 days

Film sample F-2: 50° C.×30 days

(15) Evaluation of heat-sensitive recording paper. (preparation ofrecording paper)

1 Part of amorphous silica having properties shown in Table 1 isdispersed as a filler in 4 parts of water using a dipper at 2000 rpm for5 minutes. To the mixture are added 3 parts of a solution A, 6 parts ofa solution B, 6 parts of a solution C, and 3 parts of a solution D thatare described below, followed by sufficient mixing to prepare a coatingsolution for the heat-sensitive papers. In the solutions A, B, C and D,the Paint Conditioner Model 5410 (Red Devil Inc. ) is pulverized byusing magnetic balls as a pulverizing medium, so that the averageparticle size of fine particles in the solution is smaller than 3 μm asmeasured by the Couter Counter method.

    ______________________________________                                        Solution A:  3-dibutylamino-6-7-                                                                            1     part                                                   anilinofluoran                                                                5% polyvinyl alcohol                                                                           5     parts                                     Solution B:  bisphenol A      1     part                                                   5% polyvinyl alcohol                                                                           5     parts                                     Solution C:  stearic acid amide                                                                             1     part                                                   5% polyvinyl alcohol                                                                           5     parts                                     Solution D:  zinc stearate    1     part                                                   5% polyvinyl alcohol                                                                           5     parts                                     ______________________________________                                    

A paper (paper for PPC) of 45 g/m² is coated with a coating solution forheat-sensitive recording paper prepared under the above-mentionedconditions in a coating amount of about 6 g/m² (dry basis) using acoating rod, followed by drying and calendering to obtain a paper forheat-sensitive recording.

The thus obtained heat-sensitive recording paper is evaluated withregard to its ground fouling, dynamic coloring property, and effect forpreventing the adhesion of scum.

In order to separately demonstrate the properties of the filler,furthermore the aqueous dispersion system of the amorphous silica isevaluated for its viscous properties.

(Evaluation of the heat-sensitive recording paper)

(1) Ground fouling.

The obtained heat-sensitive recording paper is left to stand in a roomfor 72 hours, and the concentration of ground fouling that spontaneouslyappears on the coated surface is measured by using Fuji StandardDensitometer FSD-103 (manufactured by Fuji Photofilm Co., Japan). At thesame time, the ground fouling is evaluated on the following basis.

◯: The concentration of ground fouling is smaller than 0.13, and almostno fouling is recognized.

Δ: The concentration of ground filing is greater than 0.13 but issmaller than 0.20, and fouling is recognized to a slight degree.

X: The concentration of ground fouling is greater than 0.20, and foulingis so conspicuous that the recording paper is not utilizable.

(2) Dynamic coloring property.

By using the heat-sensitive printing apparatus TH-PMD (manufactured byOkura Denki Co., Japan), the thus obtained heat-sensitive recordingpaper is subjected to the recording under the conditions of printingvoltage of 24 V, pulse period of 2 msec, a printing pulse width of 0.5to 1.5 msec, and a thermal head resistance of 2.651Ω, and the coloringsensitivity is evaluated on the following evaluation basis, and theaccomplished concentration (pulse width, 1.5 msec) is measured by usingthe Fuji Standard Densitometer FSD-103.

◯: A dynamic coloring sensitivity curve of coloring concentration with apulse width (0.5 to 1.5 msec) rises steeply, and the accomplishedconcentration is as high as 1.37 or more.

Δ: A dynamic coloring sensitivity curve of coloring concentration with apulse width (0.5 to 1.5 msec) rises relatively steeply, and theaccomplished concentration is greater than 1.30 but is smaller than 1.37and is relatively low.

X: A dynamic coloring sensitivity curve of coloring concentration with apulse width (0.5 to 1.5 msec) rises mildly, and the accomplishedconcentration is as low as 1.30 or smaller.

(3) Effect for preventing the adhesion of scum.

The obtained heat-sensitive recording paper is printed solidly black byusing an NTT FAX-510T, and the scum adhered to the thermal head afterthe printing is observed by naked eyes. Furthermore, a test patternpaper is prepared on which solid black squares of 1 cm×1 cm are arrangedup and down and right and left maintaining a distance of 3 mm, and isprinted on the heat-sensitive recording paper to observe by naked eyessmall black dots of scum (dispersion of scum) appearing on the whiteground (nonprinted portions) along a direction in which the recordingpaper moves. The effect for preventing the adhesion of scum is evaluatedon the following evaluation basis.

◯: Adhesion of scum on the thermal head is not at all recognized, anddispersion of scum is not recognized on the printed surface.

Δ: Adhesion of scum on the thermal head is recognized to a slightdegree, and dispersion of scum is recognized to a slight degree on theprinted surface.

X: Adhesion of scum on the thermal head is obviously recognized, anddispersion of scum is obviously recognized on the printed surface.

In order to demonstrate the properties of amorphous silica as a filler,the viscosity of an aqueous dispersion is measured under the followingconditions, and the results are shown in FIG. 4.

By using a high-speed disper, the amorphous silica is dispersed in thecity water in one-liter beakers at 2000 rpm, for 5 minutes at roomtemperature, such that the amorphous silica concentrations are 20% byweight, 30% by weight and 50% by weight. Viscosities of the dispersionslurries are,measured at 20° C. by using a B-type viscometer(manufactured by Tokyo Keiki Seizosho Co.).

EXAMPLE 1

In preparing amorphous silica using a sodium silicate solution (specificgravity 1.29, composition 3.3SiO₂.Na₂ O.nH₂ O) and a sulfuric acidsolution of a concentration of 13%, an acid silica sol was firstprepared using half the amount of the sodium silicate solution by amethod mentioned below.

The sodium silicate solution of an amount corresponding to 50% of thewhole reaction amount was poured into the sulfuric acid solution withstirring at a temperature maintained lower than 20° C. over a period of2 hours to obtain an acid silica sol having a pH of 0.7. Then, NaCl wasadded to the remaining 50% of the sodium silicate solution so that theweight ratio of SiO₂ :NaCl was 1:1, and the acid silica sol solutionobtained above was added thereto with stirring over a period of 5 hours.The obtained silica slurry that possessed pH of 6.5 was filtered andwashed, and a silica cake was dried at 110° to 350° C., pulverized andclassified to obtain amorphous silica having average secondary particlediameters of 1.8 μm, 2.7 μm and 3.5 μm (samples 1, 2 and 3). Theobtained morphous silica particles exhibited properties as shown inTable 1, the samples F-1 of resin films using the amorphous silicaparticles as AB agents were evaluated as shown in Tables 2 and 3, thesamples F-2 were evaluated as shown in Table 4, and the samples F-3 wereevaluated as shown in Table 5.

The obtained amorphous silica was 93.5% by weight reckoned as SiO₂ underthe condition of being dried at 150° C.

EXAMPLE 2

An acid silica sol was prepared in the same manner as in Example 1 usinghydrochloric acid as mineral acid, and amorphous silica was obtained ina manner as described below by using the above sol.

The sodium silicate solution of an amount corresponding to 33% of thewhole reaction amount was poured into the hydrochloric acid solution(concentration 14%, specific gravity 1.07) with stirring at atemperature maintained lower than 20° C. over a period of 1.5 hours toobtain an acid silica sol having pH of 0.3. Then, NaCl was added to theremaining 67% of the sodium silicate solution so that the weight ratioof SiO₂ :NaCl was 1: 0.15, and the mixture was heated at 60° C. To themixture was poured the acid silica sol solution that was obtained abovewith stirring over a period of 4 hours. The obtained silica slurrypossessed pH of 7.2, from which amorphous silica having an averagesecondary particle diameter of 2.18 μm was obtained in the same manneras in Example 1 (sample 4). The thus obtained amorphous silica exhibitedproperties as shown in Table 1, the sample F-1 of resin film using theamorphous silica as an AB agent was evaluated as shown in Tables 2 and3, the sample F-2 was evaluated as shown in Table 4, and the sample F-3was evaluated as shown in Table 5.

The obtained amorphous silica was 91.4% by weight reckoned as SiO₂ underthe condition of being dried at 150° C.

EXAMPLE 3

An acid silica sol was prepared in the same manner as in Example 1 usinghydrochloric acid as mineral acid, and amorphous silica was obtained ina manner as described below by using the above sol.

The sodium silicate solution of an amount corresponding to 67% of thewhole reaction amount was poured into the hydrochloric acid solution(concentration 14%, specific gravity 1.07) with stirring at atemperature lower than 20° C. over a period of 2.5 hours to obtain anacid silica sol having a pH of 1.4. Then, NaCl was added to theremaining 33% of the sodium silicate solution so that the weight ratioof SiO₂ :NaCl was 1:2.5, and the acid silica sol solution obtained abovewas added thereto with stirring over a period of 5 hours. The obtainedsilica slurry possessed a pH of 7.1, from which amorphous silica havingan average secondary particle diameter of 2.8 μm was obtained in thesame manner as in Example 1 (sample 5)- The thus obtained amorphoussilica exhibited properties as shown in Table 1, the sample F-1 of resinfilm using the amorphous silica as an AB agent was evaluated as shown inTables 2 and 3, the sample F-2 was evaluated as shown in Table 4, andthe sample F-3 was evaluated as shown in Table 5.

The obtained amorphous silica was 94.6% by weight reckoned as SiO₂ underthe condition of being heated at 150° C.

EXAMPLE 4

In preparing amorphous silica, a concentrated acid silica sol solutionwas continuously obtained by a method described below and, then, asodium silicate solution containing NaCl and an acid silica sol solutionwere continuously contacted and reacted together under a heatedcondition to obtain amorphous silica.

An acid silica sol was prepared by a method described below using thesame sodium silicate solution as that of Example 1 and a sulfuric acidsolution (concentration 40%, specific gravity 1.25). The sodium silicatesolution of an amount corresponding to 50% of the whole reaction amountand the sulfuric acid solution were continuously supplied at atemperature of lower than 25° C. using an apparatus capable of supplyingthem at a volume ratio of 4:1, and were quickly shear-stirred in orderto continuously obtain an acid silica sol (pH 2.1). To the remaining 50%of the sodium silicate solution was added NaCl in such an amount thatthe weight ratio of SiO₂ :NaCl was 1:2. The resulting sodium silicatesolution and the acid silica sol solution obtained by the above methodwere continuously supplied and were reacted together at 60° C. withrapid shear-stirring to obtain amorphous silica, from which amorphoussilica having an average secondary particle diameter of 2.2 μm (sample6) and amorphous silica having an average secondary particle diameter of4.1 μm (sample 7) were obtained in the same manner as in Example 1. Thethus obtained amorphous silica particles exhibited properties as shownin Table 1. The thus obtained amorphous silica particles exhibitedproperties as shown in Table 1, the samples F-1 of resin films using theamorphous silica particles as AG agents were as evaluated in Tables 2and 3, the samples F-4 were as evaluated in Table 4, and the samples F-3were as evaluated in Table 5.

The obtained amorphous silica was 92.8% by weight reckoned as SiO₂ underthe condition of being heated at 150° C.

(Comparative Example 1)

The film was tested and evaluated in the same manner as in the aboveExamples by using a sample H1 of amorphous silica which was acommercially available product A (produced by Mizusawa Kagaku Kogyo Co.,Japan) having an average secondary particle diameter of 1.7 μm.

(Comparative Example 2)

The film was tested and evaluated in the same manner as in the aboveExamples by using a sample H2 of amorphous silica which was acommercially available product B (produced by. Fuji Debison Co., Japan)having an average secondary particle diameter of 2.1 μm.

(Comparative Example 3)

The film was tested and evaluated in the same manner as in the aboveExamples by using a sample H3 of amorphous silica which was acommercially available product B (produced by Fuji Debison Co.) havingan average secondary particle diameter of 3.1 μm.

Like in the above Examples, the amorphous silica particles obtained inthe Comparative Examples 1, 2 and 3 exhibited properties as shown inTable 1, the samples F1 of resin films using the amorphous silicaparticles as AB agents were evaluated as shown in Tables 2 and 3, thesamples F-2 were evaluated as shown in Table 4, and the samples F-3 wereevaluated as shown in Table 5.

(EXAMPLE 5 and 6)

The amorphous silica particles of samples Nos. 1 and 6 obtained inExamples 1 and 4 were used as fillers for heat-sensitive recordingpapers, in order to prepare heat-sensitive recording papers by theaforementioned recording paper preparation method. The ground fouling,dynamic coloring property, and effect for preventing the adhesion ofscum were evaluated. The results were as shown in Table 6.

Comparative Examples 4 to 6

The amorphous silica particles of samples Nos. H1 to H3 used inComparative Examples 1 to 3 were used as fillers for heat-sensitiverecording papers, in order to prepare recording papers in the samemanner as in Example 5. The results were as shown in Table 6.

                                      TABLE 1                                     __________________________________________________________________________    Properties of amorphous silica particles                                                Example                     Comparative Example                               1   1   1   2   3   4   4   1   2   3                               Sample No.                                                                              1   2   3   4   5   6   7   H1  H2  H3                              __________________________________________________________________________    pH        7.14                                                                              7.15                                                                              7.14                                                                              7.50                                                                              7.8 6.7 6.72                                                                              6.98                                                                              7.96                                                                              3.90                            Oil-sucking                                                                             86  83  76  92  98  70  63  162 263 106                             amount(ml/100 g)                                                              Specific surface                                                                        370 376 386 315 280 465 465 55  288 704                             area (m.sup.2 /g)                                                             Apparent specific                                                                       0.321                                                                             0.342                                                                             0.376                                                                             0.286                                                                             0.272                                                                             0.43                                                                              0.523                                                                             0.188                                                                             0.138                                                                             0.506                           gravity (g/cm.sup.3)                                                          Average secondary                                                                       1.8 2.7 3.5 2.1 2.8 2.2 4.1 1.7 2.1 3.1                             particle diameter                                                             (μm)                                                                       Average primary                                                                         180 180 180 150 200 250 250 80  9.8*                                                                              ∞                         particle diameter:                                                            D1 (nm)                                                                       Elementary                                                                              7.4 7.3 7.1 8.7 9.7 5.9 5.9 49.6                                                                              9.4 3.8                             particle diameter:                                                            D0 (nm)                                                                       Agglomeration                                                                           24.4                                                                              24.8                                                                              25.5                                                                              17.3                                                                              20.5                                                                              42.7                                                                              42.7                                                                              1.61                                                                              1.04                                                                              ∞                         degree:                                                                       (D1/D0 = DA)                                                                  __________________________________________________________________________     *Average diameter of particles from which coarse particles as seen in FIG     2 have been removed.                                                     

                                      TABLE 2                                     __________________________________________________________________________    Evaluation of film samples F-1                                                          Example              Comparative Example                                      1  1  1  2  3  4  4  1   2  3                                       Sample No.                                                                              1  2  3  4  5  6  7  H1  H2 H3                                      __________________________________________________________________________    Haze (%)  2.5                                                                              2.6                                                                              2.8                                                                              2.5                                                                              2.7                                                                              2.5                                                                              3.2                                                                              2.5 2.7                                                                              2.9                                     Gloss (%) 97 95 95 96 95 96 93 97  95 93                                      Slipping property*                                                            static/   0.65/                                                                            0.68/                                                                            0.70/                                                                            0.65/                                                                            0.70/                                                                            0.63/                                                                            0.72/                                                                            0.60/                                                                             0.65/                                                                            0.68/                                   dynamic   0.51                                                                             0.53                                                                             0.55                                                                             0.50                                                                             0.56                                                                             0.50                                                                             0.58                                                                             0.50                                                                              0.50                                                                             0.55                                    Slipping property*.sup.2                                                      static/   0.31/                                                                            0.33/                                                                            0.34/                                                                            0.29/                                                                            0.34/                                                                            0.28/                                                                            0.36/                                                                            0.27/                                                                             0.30/                                                                            0.33/                                   dynamic   0.27                                                                             0.29                                                                             0.30                                                                             0.26                                                                             0.30                                                                             0.25                                                                             0.32                                                                             0.24                                                                              0.26                                                                             0.29                                    Anti-blocking                                                                           good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             0.25                                                                              good                                                                             good                                    property (g/cm)*.sup.3                                                        Scratch-resistant                                                                       ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                    ◯                                                                    ◯                                                                    ◯                                                                    ◯                                                                     Δ                                                                          X                                       property                                                                      Appearance of film                                                                      ⊚                                                                 ⊚                                                                 ◯                                                                    ⊚                                                                 ⊚                                                                 ⊚                                                                 ◯                                                                    X   X  ⊚                        __________________________________________________________________________     *.sup.1 23° C. × 1 day,                                          *.sup.2 40° C. × 3 days,                                         *.sup.3 50° C. × 7 days                                     

                                      TABLE 3                                     __________________________________________________________________________    Evaluation of film samples F-1                                                          Example              Comparative Example                                      1  1  1  2  3  4  4  1   2  3                                       Sample No.                                                                              1  2  3  4  5  6  7  H1  H2 H3                                      __________________________________________________________________________    Haze (%)  2.0                                                                              2.6                                                                              3.2                                                                              2.0                                                                              2.8                                                                              2.0                                                                              3.6                                                                              2.1 2.9                                                                              2.6                                     Gloss (%) 110                                                                              100                                                                              96 110                                                                              100                                                                              110                                                                              97 120 100                                                                              96                                      Slipping property*                                                            static/   0.30/                                                                            0.35/                                                                            0.45/                                                                            0.32/                                                                            0.39/                                                                            0.32/                                                                            0.48/                                                                            0.27/                                                                             0.41/                                                                            0.37/                                   dynamic   0.26                                                                             0.32                                                                             0.37                                                                             0.27                                                                             0.31                                                                             0.27                                                                             0.37                                                                             0.24                                                                              0.32                                                                             0.32                                    Slipping property*.sup.2                                                      static/   0.13/                                                                            0.20/                                                                            0.27/                                                                            0.12/                                                                            0.22/                                                                            0.15/                                                                            0.27/                                                                            0.12/                                                                             0.12/                                                                            0.24/                                   dynamic   0.12                                                                             0.18                                                                             0.25                                                                             0.11                                                                             0.20                                                                             0.14                                                                             0.25                                                                             0.11                                                                              0.11                                                                             0.23                                    Anti-blocking                                                                           1.25                                                                             1.00                                                                             0.36                                                                             1.26                                                                             0.95                                                                             1.15                                                                             0.32                                                                             2.95                                                                              1.26                                                                             1.29                                    property (g/cm)*.sup.3                                                        Scratch-resistant                                                                       ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                    ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                     Δ                                                                          X                                       property                                                                      Appearance of film                                                                      ⊚                                                                 ⊚                                                                 ◯                                                                    ⊚                                                                 ◯                                                                    ⊚                                                                 ◯                                                                    X   X  ⊚                        __________________________________________________________________________     *.sup.1 23°  C. × 1 day,                                         *.sup.2 40° C. × 3 days,                                         *.sup.3 50° C. × 7 days                                     

                                      TABLE 4                                     __________________________________________________________________________    Evaluation of film samples F-2                                                          Example              Comparative Example                                      1  1  1  2  3  4  4  1   2  3                                       Sample No.                                                                              1  2  3  4  5  6  7  H1  H2 H3                                      __________________________________________________________________________    Haze (%)  1.0                                                                              1.1                                                                              1.3                                                                              1.0                                                                              1.3                                                                              1.0                                                                              1.4                                                                              1.1 1.3                                                                              1.4                                     Gloss (%) 143                                                                              139                                                                              135                                                                              140                                                                              139                                                                              140                                                                              133                                                                              143 135                                                                              133                                     Slipping property*.sup.1                                                      static/   0.52/                                                                            0.51/                                                                            0.48/                                                                            0.52/                                                                            0.51/                                                                            0.52/                                                                            0.45/                                                                            0.58/                                                                             0.51/                                                                            0.51/                                   dynamic   0.52                                                                             0.50                                                                             0.47                                                                             0.52                                                                             0.50                                                                             0.51                                                                             0.43                                                                             0.55                                                                              0.50                                                                             0.49                                    Anti-blocking                                                                           good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             0.13                                                                              0.05                                                                             good                                    property (g/cm)*.sup.2                                                        Scratch-resistant                                                                       ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                    ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                     Δ                                                                          X                                       property                                                                      Appearance of film                                                                      ⊚                                                                 ⊚                                                                 ◯                                                                    ⊚                                                                 ◯                                                                    ⊚                                                                 ◯                                                                    ⊚                                                                  X  X                                       __________________________________________________________________________     *.sup.1 40° C. × 1 day,                                          *.sup.2 50° C. × 30 days                                    

                                      TABLE 5                                     __________________________________________________________________________    Evaluation of film samples F-3                                                          Example              Comparative Example                                      1  1  1  2  3  4  4  1   2  3                                       Sample No.                                                                              1  2  3  4  5  6  7  H1  H2 H3                                      __________________________________________________________________________    Haze (%)  1.6                                                                              2.1                                                                              2.8                                                                              1.7                                                                              2.2                                                                              1.7                                                                              2.8                                                                              5.0 2.4                                                                              2.2                                     Gloss (%) 143                                                                              139                                                                              135                                                                              140                                                                              139                                                                              145                                                                              135                                                                              152 140                                                                              135                                     Slipping property*.sup.1                                                      static/   0.36/                                                                            0.42/                                                                            0.50/                                                                            0.35/                                                                            0.51/                                                                            0.36/                                                                            0.53/                                                                            0.51/                                                                             0.39/                                                                            0.43/                                   dynamic   0.35                                                                             0.40                                                                             0.45                                                                             0.35                                                                             0.47                                                                             0.35                                                                             0.44                                                                             0.40                                                                              0.37                                                                             0.40                                    Slipping property*.sup.2                                                      static/   0.46/                                                                            0.51/                                                                            0.57/                                                                            0.45/                                                                            0.58/                                                                            0.50/                                                                            0.55/                                                                            0.57/                                                                             0.60/                                                                            0.68/                                   dynamic   0.43                                                                             0.47                                                                             0.47                                                                             0.42                                                                             0.50                                                                             0.48                                                                             0.47                                                                             0.47                                                                              0.55                                                                             0.62                                    Scratch-resistant                                                                       ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                    ◯                                                                    ◯                                                                    Δ                                                                          ◯                                                                     Δ                                                                          X                                       property                                                                      Appearance of film                                                                      ⊚                                                                 ⊚                                                                 ◯                                                                    ⊚                                                                 ◯                                                                    ⊚                                                                 ◯                                                                    ◯                                                                     X  X                                       __________________________________________________________________________     *.sup.1 24° C. × R.H. 65%,                                       *.sup.2 25° C. × R.H. 85%                                   

                                      TABLE 6                                     __________________________________________________________________________                          Example Comparative Example                                                   5   6   4    5    6                                     Sample No.            1   6   H1   H2   H3                                    __________________________________________________________________________    Properties of amorphous silica particles                                      pH                    7.14                                                                              6.70                                                                              6.98 7.96 3.90                                  Dispersion property (μm)                                                                         10  10  15-20                                                                              12-16                                                                              9-12                                                        or less                                                                           or less                                             Average particle diameter (μm)                                                                   1.8 2.2 1.7  2.1  3.1                                   Average primary particle                                                                            180 250 80   9.8* ∞                               diameter D1 (nm)                                                              Oil-sucking amount (ml/100 g)                                                                       86  70  162  263  106                                   Specific surface area (m.sup.2 /g)                                                                  370 465 55   288  704                                   Apparent specific gravity                                                                           0.321                                                                             0.430                                                                             0.188                                                                              0.139                                                                              0.506                                 (g/cm.sup.3)                                                                  Calculated primary particle                                                                         7.37                                                                              5.86                                                                              49.58                                                                              9.46 3.8                                   diameter D0 (nm)                                                              Agglomeration degree (D1/D0)                                                                        24.4                                                                              42.66                                                                             1.61 1.03 ∞                               Evaluation of heat-sensitive recording papers                                 Ground fouling                                                                Evaluation            ◯                                                                     ◯                                                                     ◯                                                                      X    X                                     Concentration         0.12                                                                              0.12                                                                              0.11 0.32 0.36                                  Dynamic coloring property                                                     Evaluation of coloring                                                                              ◯                                                                     ◯                                                                     ◯                                                                      ◯                                                                      ◯                         sensitivity                                                                   Accomplished          1.40                                                                              1.38                                                                              1.40 1.41 1.41                                  concentration                                                                 Effect for preventing ◯                                                                     ◯                                                                     ◯                                                                      ◯                                                                      ◯                         adhesion of scum                                                              Viscosity                                                                     viscosity of 20 wt % aqueous                                                                        5   5   10   260  not                                   dispersion slurry (CPS)                 dispersed                             viscosity of 30 wt % aqueous                                                                        7   6   113  not  not                                   dispersion slurry (CPS)            dispersed                                                                          dispersed                             viscosity of 50 wt % aqueous                                                                        35  30  not  not  not                                   dispersion slurry (CPS)       dispersed                                                                          dispersed                                                                          dispersed                             __________________________________________________________________________     *Value of only those particles whose diameters could be measured.        

We claim:
 1. A heat sensitive recording paper coated with a layer comprising a composition containing a silica filler, wherein said silica filler comprises amorphous silica particles which have an average primary particle diameter (D1) of from 100 to 270 nm, an apparent specific gravity of from 0.24 to 0.55 g/cm³, a specific surface area of from 200 to 500 m² g, a silica elementary particle diameter (DO) of from 5 to 15 nm, and an agglomeration degree (DA) defined by the ratio of D1/DO of from 10 to 50, wherein said coating layer composition contains said silica filler in an amount of 5 to 60% by weight based on the weight of the solid content of the coating layer composition.
 2. A heat sensitive recording paper according to claim 1, wherein the silica filler comprises amorphous silica particles which have a secondary particle (D2) median diameter of 1 to 5 μm, and exhibit an oil absorption capacity of 50 to 120 ml/100 g.
 3. A heat sensitive recording paper according to claim 1 wherein the silica filler comprises an aqueous dispersion slurry of said amorphous silica particles containing 20 to 50% by weight of said amorphous silica particles, wherein said aqueous dispersion has a viscosity of 1 to 100 cps, measured at 20° C.
 4. A heat sensitive recording paper according to claim 2 wherein the silica filler comprises an aqueous dispersion slurry of said amorphous silica particles containing 20 to 50% by weight of said amorphous silica particles, wherein said aqueous dispersion has a viscosity of 1 to 100 cps, measured at 20° C.
 5. A heat sensitive recording paper having a heat sensitive recording layer composition containing a silica filler, wherein said silica filler comprises amorphous silica particles which have an average primary particle diameter (D1) of from 100 to 270 nm, an apparent specific gravity of from 0.24 to 0.55 g/cm³, a specific surface area of from 200 to 500 m² g, a silica elementary particle diameter (DO) of from 5 to 15 nm, and an agglomeration degree (DA) defined by the ratio of D1/DO of from 10 to 50, and wherein said recording layer composition contains said silica filler in an amount of 5 to 60% by weight based on the weight of the solid content of the recording layer composition.
 6. A heat sensitive recording paper according to claim 5 wherein the silica filler comprises amorphous silica particles which have a secondary particle median diameter (D2) of 1 to 5 μm and exhibit an oil absorption capacity of 50 to 120 ml/¹⁰⁰ g.
 7. A heat sensitive recording paper according to claim 5 wherein the silica filler comprises an aqueous dispersion slurry of said amorphous silica particles containing 20 to 50% by weight of said amorphous silica particles, wherein said aqueous dispersion has a viscosity of 5 to 100 cps, measured at 20° C.
 8. A heat sensitive recording paper according to claim 6 wherein the silica filler comprises an aqueous dispersion slurry of said amorphous silica particles containing 20 to 50% by weight of said amorphous silica particles, wherein said aqueous dispersion has a viscosity of 5 to 100 cps, measured at 20° C. 